examples/ipsec-secgw: fix --mtu option parsing
examples/ipsec-secgw: fix --mtu option parsing

This patch add parameter --mtu parse key and enumeration value.

Fixes: b01d1cd213d2 ("examples/ipsec-secgw: support fragmentation and reassembly")

Signed-off-by: Tao Zhu <98fc9452bad001bbed4b53169990d94b082d6178@intel.com>
Acked-by: Konstantin Ananyev <cbd5212b59ab42a210d72992d23a3aa17cfd3eaf@intel.com>
Tested-by: Konstantin Ananyev <cbd5212b59ab42a210d72992d23a3aa17cfd3eaf@intel.com>

john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk,john-mcnamara-intel/dpdk